Archive for the 'Chelicerates' Category

The rather amusing cover of this month’s JEB caught my eye; I am always excited to find out about the outlandish and creative methods that scientists dream up in order to test their ideas.

SCIENCE!

Yep, that’s a jumping spider holding a styrofoam ball, tethered to the ceiling. So what the heck could possibly be going on here?

The cover shot belongs to this paper. The researchers wanted to get a better handle on the contributions of specific jumping spider eye sets to the animal’s overall visual perception and behavior. Like many arachnids, jumping spiders have eight corneal eyes. Two sets of these eyes are forward facing; the anterior median (AM) and anterior lateral (AL) eyes (see image below). The large set of AM eyes are extremely acute, boasting the highest known resolution among the arthropods. However, they have an extremely narrow field of view since their retina is organized into a thin strip, not unlike the ribbon retina of larval diving beetles. The AL eyes, on the other hand, have a much larger, overlapping field of view and are very good at detecting movement. When the spider detects something with the AL eyes, it reorients its body to bring the high-resolution AM eyes to bear on the target. When jumping spiders are active they can be seen constantly preforming these reorienting body movements, endowing them with a great deal of inquisitive charm and personality (adorable videos).

Now, back to the recently published study. The researchers wanted to assess the importance of the AL eyes in the orientation response of jumping spiders. They used an opaque silicone paint to block out all the animal’s eyes besides the two AL eyes. They then tethered the jumping spiders from above using a piece of cork and beeswax. Finally they ‘handed’ the spiders a gridded polystyrene sphere (which they readily accepted), and positioned them in front of computer LCD monitors. Varying dot stimuli were displayed on the monitors, and the orientation response of the spiders to these stimuli were easily recorded by observing the underfoot movements of the polystyrene sphere.

The researchers found that the jumping spider’s AL eyes are crucial to orientation responses, and therefore extremely important to the spider’s visual ecology. In fact, the spiders in this study demonstrated complete hunting behaviors using only the AL eyes. In addition, the researchers noted that increased hunger yielded stronger predatory response in the jumping spiders. Finally, they observed that overall, females showed a greater orientation response to stimuli than males. The researchers suggest that this is due to visual dimorphism, possibly related to the female’s need to carefully scrutinize the courting displays preformed by males.

So, that’s why the cover of JEB is a photo of a hanging jumping spider holding a polystyrene ball. However, the best part of this outlandish-seeming experiment is that the tests were non-destructive. The paint covering the eyes, and the tether attached to the back, could be removed without harming the jumping spiders. They were, unfortunately, eventually forced to give up their toy ball.
_

Here is yet another arthropod photo that gets a lot of attention around the internet (and in hysterical mass emails from your aging relatives). During the recent deployments, US servicemen started running into these unsettling arachnids, commonly called camel-spiders, in the Iraqi desert. They subsequently sent home a good deal of photos, rumors, and urban legends about them. To the left is the most popular camel-spider photo that is circulated around the web. Claims about these arachnids include (according to Snopes.com):

Most Solifugids are highly specialized for survival in arid habitats and they are found in deserts around the world, excluding Australia. They are mostly nocturnal to avoid the heat, but some species are diurnal. Shade is crucial to the survival of arid solifugids that are active during the day. The reports of camel spiders charging and pursuing soldiers are likely derived from the animals attempting to take refuge from the sun in their shadows. As far as the shrieking sound that they allegedly make during their charges: Perhaps the sound actually comes from the men, who later try to save face in front of their buddies by attributing the noise to the arachnid. I can relate.

One of the most obvious physical characteristics of solifugids are their massively enlarged chelicerae. These appendages give the impression of tremendously engorged, venom-laden fangs. However, their size is actually a compensation for a lack of venom (There is a single Indian species, Rhagodes nigrocinctus that may possess venom glands, but this has not been well confirmed and there is no known injection mechanism). Each of the chelicerae are composed of two segments forming powerful pincers. These pincers are used to grasp and tare apart their prey; which includes other arthropods, lizards, snakes, and possibly small mammals. Solifugids do not feed on animals larger than themselves, and they do not munch away on humans or camels, unnoticed through the use of anesthetic venom. If these guys take a chomp out of you, you will notice. However, they are not particularly aggressive to people unless harassed or backed into a corner.

Unlike spiders and scorpions, solifugids superficially appear to possess ten legs. However, the largest, foremost ‘legs’ are actually enlarged, antennae-like, sensory appendages, called pedipalps. The pedipalps are also used for climbing and prey capture.

Additionally, the first set of true legs are also used as accessory sensory appendages, leaving only the back three sets of legs for locomotion. Solifugids are nonetheless capable of quick bursts of speed (up to 53 cm/sec or 1.2 mph) when attacking prey or darting for cover. However, the claims that they can travel at 25 mph and keep pace with Humvees are obvious exaggerations.

Now, to address the ‘size of dinner plates’ claim about solifugids, which is reinforced by the popular image at the top of the article: Amazingly, for these sorts of meme inducing images, I was actually able to track down the photo’s origin. According to Paula Cushing, Department Chair and Curator of Invertebrate Zoology at the Denver Museum of Nature and Science, the photo was taken by a serviceman and amateur photographer named John Sellers. He photographed the two animals, one violently clamped onto the others abdomen, following a gladiatorial battle between the two solifugids staged by the soldiers (an ugly practice that has gone on at least since British troops were stationed in Egypt during the first World War, and continues today in Youtube videos). Though I was unable to determine an exact species ID, the pictured solifugids are members of the Galeodes genus (perhaps G. granti or G. arabs?). They are about 10 cm in total length, residing at the upper end of the solifugid size spectrum, which ranges from 1 cm to 10 cm in body length. The largest example of a solifugid I could dig up is this Galeodes fumigatus, which appears to be about 11 cm or more in body length. So these animals can reach a menacing size, but nothing close to a dinner plate, or the size suggested by the tricky perspective of the camel-spider meme photo.

Some solifugids can be large and creepy-looking, to be sure, but they are not the deadly, bloodthirsty, lightning-fast, venom-dripping, monsters that popular culture has portrayed them as. If you can redirect fear and revulsion into fascination, and look beyond the unsettling facades of monstrous arthropods, you may find a vibrant wealth of astonishing realities.
–

Put yourself in my place: You’re collecting mantis shrimp by cracking reef rubble on the beach of an island on the Great Barrier Reef (I know, there are worse ways to spend your day). You split one particular rock, and instead of shrimp, out pours a brood of arachnids. Not pycnogonids, not spider crabs… freaking spiders! Well, at this point, if you’re me, you shriek like a little girl and frantically crab-walk backwards while brushing yourself off.

In that embarrassing moment I had learned something new: There are arachnids that live, partially, in the ocean. These small Desid spiders live in intertidal rubble. During the day they hide in silk-sealed air chambers within the rubble. At low tide they come out to hunt stranded critters along the tide pools.

A marine spider, Desis martensi, on an out-of-water coral head. It has captured what looks like a pistol shrimp. Photo: Wild Singapore

Check out this gallery of false-colored scanning electron micrographs of insects and arachnids by Steve Gschmeissner. There is so much detail to the portraits of these every-day critters that we don’t usually get to see.

New fossil arachnids of the order Araneae from Inner Mongolia, China have been described in the journal, Naturwissenschaften.Eoplectreurys gertschi fossils. The body length of these tiny spiders is only about 3 mm, excluding the chelicerae.

These fossils are remarkable for a variety of reasons:

First, they are very well preserved, allowing researchers to examine minute details of this extinct species (See image below).

Secondly, the fossils are extremely old, from the Middle Jurassic (165 million years ago). They extend the fossil record of the family Plectreuridae back 120 million years beyond previous specimens. The morphological conservation between these fossil Plectreuroids and their modern counterparts displays strong evolutionary conservatism (the creationists are gonna’ love that detail).

Finally, the discovery of the fossils in China supports the hypothesis that this spider family once had a much larger range. Today they are only found in the US, Mexico, and the Carribean.

ABC news has a story about a tarantula hair attack. You would think that the venom laden fangs were enough to worry about, but it turns out that some species can also launch a mist of microscopic, barbed urticating hairs into your eyes, skin, and lungs; causing intense irritation.

A British man discovered this defensive behavior the hard way. While cleaning the cage of his Chilean Rose tarantula, Grammostola rosea, the man was hit in the face by a fine mist of hairs from the arachnid. He though nothing of it at first, but over a week the barbed hairs in his eye worked their way into his cornea, iris, and even to the retina at the back of the eye. This resulted in extreme irritation that baffled doctors until they spotted the microscopic hairs and the pet-owner remembered the hair misting incident.

This defensive behavior is common in New World tarantulas and some caterpillars. In tarantulas, the urticating hairs are located on the dorsal abdomen, or the palps in the case of the genus Ephebopus. When threatened, they brush the urticating hairs off or their body with rapid leg-flicking motions. Here is a quick video of the behavior (loud soundtrack warning).

Brachypelma smithi tarantula with a bare patch on its abdomen where it has scraped off its urticating hairs during a defense response (left frame). Scanning electron micrograph (SEM) of a Ephebopus palpal urticating hair region after brushing off most of the hairs in a defense response (right frame). A couple barbed urticating hairs are still in their sockets (Foelix et al., 2009).